Dermatopathologic Variants of Malignant Melanoma

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An appropriate biopsy is the pivotal procedure that facilitates accurate histopathologic diagnosis of a pig mented skin lesion. Excisional skin biopsy is the method of choice for removing a suspected malignant melanoma. More than 95% of malignant melanomas that involve the skin belong to one of the four most common clinicopathologic categories: superficial spreading, nodular, lentigo maligna, and acral lentiginous melanoma. A small but important group of cutaneous melanomas can be classified as unusual variants. Many of these unusual variants have a distinct histopathologic appearance; they include desmoplastic melanoma, neurotropic melanoma, pedunculated melanoma, metastatic melanoma, amelanotic melanoma, melanoma arising within a benign nevus, regressing (“invisible”) melanoma, and balloon cell melanoma. Other lesions may simulate malignant melanoma histopathologically. Immunohistochemical stains, such as S – 100 protein, vimentin, keratin, and HMB – 45, are useful for distinguishing these lesions from true melanoma.

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BIOPSY TECHNIQUE

Ideally, all biopsy procedures for malignant melanoma would accomplish complete excision of the lesion to the depth of subcutaneous tissue. Melanoma, however, is not always suspected clinically. In particular, this situation prevails with unusual variants of melanoma, such as desmoplastic or amelanotic lesions. Nonetheless, when melanoma is considered in the differential diagnosis, an excisional biopsy is recommended. This approach allows adequate distinction of the neoplasm from the

HISTOPATHOLOGIC CLASSIFICATION OF MALIGNANT MELANOMA

Superficial spreading malignant melanoma, lentigo maligna melanoma, nodular melanoma, and acral lentiginous melanoma account for most cases of malignant melanoma. The remaining melanomas (less than 5% of cases) are unusual histopathologic subtypes.

Desmoplastic malignant melanoma is probably the most common of the unusual melanoma subtypes. As its name implies, desmoplastic melanoma occurs in conjunction with a fibrotic connective tissue stroma. Most often, desmoplastic melanoma arises in the

IMMUNOHISTOCHEMISTRY

S-100 protein is a 21-kd protein that was extracted originally from bovine brain in 1965.24 Since then, S-100 protein has been noted in the peripheral nerves and central nervous system of many vertebrates and in melanocytes and other isolated cells, such as Langerhans' cells.25 S-100 protein stains 96.4% of malignant melanomas and 98.4% of benign melanocytic nevi.26 In addition, it stains 15.1 % of nonmelanocytes,26 primarily neural and Langerhans' cells. S-100 protein is a sensitive

SIMULATORS OF MALIGNANT MELANOMA

Several benign and malignant neoplasms simulate malignant melanoma and can cause difficulty for pathologists or dermatopathologists. Spitz nevi usually are sharply circumscribed but may have a substantial intraepidermal component. Eosinophilic globules (Kamino bodies) may be present in the papillary dermis, a finding that can be an aid to diagnosis.34 A variant of the Spitz nevus, the pigmented spindle cell nevus of Reed,17 is sharply demarcated but heavily pigmented and shows a preponderance

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